1. Field of the Invention
The invention relates generally to friction stir welding of high melting temperature materials. More specifically, the present invention teaches a method and apparatus for joining high melting temperature materials in a friction stir welding process, without substantially changing the macrostructure and the microstructure of those materials.
2. Description of Related Art
Friction stir welding (hereinafter referred to as “FSW”) is a technology that has been developed for welding metals, metal alloys, and other materials. The friction stir welding process often involves engaging the material of two adjoining workpieces on either side of a joint by a rotating stir pin or spindle. Force is exerted to urge the spindle and the workpieces together and frictional heating caused by the interaction between the spindle and the workpieces results in plasticization of the material on either side of the joint. The spindle is traversed along the joint, plasticizing material as it advances, and the plasticized material left in the wake of the advancing spindle cools and solidifies to form a weld.
It will be appreciated that large forces must be exerted between the spindle and the workpieces in order to apply sufficient pressure to the workpieces to cause plasticization of the material. For instance, for friction stir welding an aluminum alloy workpiece of ¼-inch thickness, forces of up to 4000 pounds or more may have to be exerted between the spindle and the workpiece. In a conventional friction stir welding process, these large forces are absorbed at least partially by a back-up member or anvil which engages the workpieces on the “back side” of the weld opposite the spindle. Where the workpieces have sufficient structural strength and rigidity, some of the force may be absorbed by the workpieces themselves. However, in many cases the workpieces are semi-flexible structures that are incapable of Supporting and absorbing the large forces involved in a friction stir welding process. Accordingly, the back-up member is usually supported by a substantial support structure.
Another example of friction welding occurs when the ends of two pipes are pressed together while one pipe is rigidly held in place, and the other is pressed against it and turned. As heat is generated by friction, the ends of the pipes become plasticized. By quickly stopping rotation of the pipes, the two pipes fuse together. Note that in this case, the frictional heating is caused by the relative motion of the two parts to be joined.
FIG. 1 is a perspective view of a tool being used for friction stir butt welding that is characterized by a generally cylindrical tool 10 having a shoulder 12 and a pin 14 extending outward from the shoulder. The pin 14 is rotated against a workpiece 16 until sufficient heat is generated, at which point the pin of the tool is plunged into the plasticized workpiece material. The workpiece 16 is often two sheets or plates of material that are butted together at a joint line 18. The pin 14 is plunged into the workpiece 16 at the joint line 18.
The frictional heat caused by rotational motion of the pin 14 against the workpiece material 16 causes the workpiece material to soften without reaching a melting point. The tool 10 is moved transversely along the joint line 18, thereby creating a weld as the plasticized material flows around the pin from a leading edge to a trailing edge. The result is a solid phase bond 20 at the joint line 18 that is generally indistinguishable from the workpiece material 16 itself.
A number of prior art friction stir welding patents disclose various tooling and techniques to obtain welds that have beneficial characteristics over contemporary fusion welding processes. These benefits include low distortion in long welds, no fumes, no porosity, no splatter, and excellent mechanical properties regarding tensile strength. The process is especially useful for preventing significant heat damage or otherwise altering the properties of the original material being welded.
However, while friction stir welding is a very advantageous technique for welding non-ferrous alloys such as aluminum, brass and bronze, typical prior art friction stir welding tools are not capable of functionally welding materials having higher melting points. It should be understood that functionally weldable materials are those that are weldable using friction stir welding in more than nominal lengths, and without destroying the tool.
A particular type of material that would be desirable to friction stir welding and which has broad industrial applications, are ferrous alloys. Ferrous alloys include steel and stainless steel. Another class of materials that would be desirable to friction stir weld, have broad industrial applications, have a higher melting point than ferrous alloys, and either have a small amount of iron or none, are the superalloys. Superalloys are nickel-, iron-nickel, and cobalt-base alloys generally used at temperatures above 1000 degrees F. Additional elements commonly found in superalloys include, but are not limited to, chromium, molybdenum, tungsten, aluminum, titanium, niobium, tantalum, and rhenium.
It is noted that titanium is also a desirable material to friction stir weld. Titanium is a non-ferrous material, but has a higher melting point than other nonferrous materials.
These materials, which are described herein as “high melting temperature materials,” have particular use in the energy industry, which uses long segments of pipe (typically broken into 40 foot segments for shipping purposes), which must then be assembled into longer segments.
As noted above, prior methods and apparatuses for friction stir welding have been unable to join these high melting temperature materials. Accordingly, more traditional methods such as plasma welding have been used to join these materials. A state of the art prior art approach starts with steel plate that is pressed in several stages into a round tube so that the edges touch, then uses double submerged arc welding (DSW) arrayed generally in a gang of welding heads to weld the edges together along the axis, generally in 40 foot sections. Metalurgically, however, this process changes the microstructure of the steel, which can cause failure of the welds in certain applications. In particular, this method will convert the steel from a wrought microstructure to a cast microstructure.
What is needed, therefore, are techniques for joining these high melting temperature materials, without substantially changing the macrostructure and/or the microstructure of the materials, as these changes tend to result in poor performance.